My experiments trying to reduce size of scanned pdf with 100+ pages (which breaks graphicsmagick convert).
./pdf-to-bilevel.pl file.pdf
This will resize your pdf into fixed dpi and in my expirience is less useful than converting it to black and white, especially on poor scans
Usage: ./pdf-optimize.sh file.pdf [screen]
# screen low-resolution "Screen Optimized" 96 dpi
# ebook medium-resolution "eBook" 150 dpi
# printer output "Print Optimized" 300 dpi
# prepress output "Prepress Optimized"
# default
# kindle - special conversion to grayscale, 150 dpi